As a consequence of the ongoing reduction of the stratospheric ozone layer, the vegetation is exposed to increasing levels of UV‐B radiation (280–320 nm). In addition ozone in the troposphere is a pollutant and also capable of affecting the photosynthetic machinery. In this study, 5‐year‐old European beech trees were exposed from 1 July to October 1993 to two levels of UV‐B radiation and two levels of ozone, alone and in combination, in open‐top chambers equipped with lamps. The simulated UV‐B levels corresponded to either clear sky ambient level or a 14% decrease in the stratospheric ozone column over eastern Denmark, resulting in a 23% difference in biologically effective UV‐B (UV‐BBE) irradiance. The maximum UV‐Bbe given was 8.61 kJ m−2 day−1. The ozone levels were either the ambient (average 32 nl l−1) or ambient with ozone addition (average resulting concentration 71 nl l−1). Compared to the control treatment (ambient UV‐B, ambient O3) the elevated levels of UV‐B and O3 affected the trees negatively, expressed as declines in net photosynthesis (Pn), stomatal conductance (gs), chlorophyll fluorescence (Fv/Fm) and acceleration of senescence, measured as yellowing of the leaves. The UV‐B treatment induced stomatal closure before the other treatments did. The magnitude of the decreases in Pn and Fv/Fm occurred in the order: control

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Introduction Zeuthen, J., Mikkelsen, T N., Paludan-Miiller, G. and Ro-Poulsen, H. 1997. Effects of increased UV-B radiation and elevated levels of tropospheric ozone on physiological processes in European beech (Eagus sylvatica). -Physiol. Plant. 100: 281-290.As a consequence of the ongoing reduction of the stratospheric ozone layer, the vegetation is exposed to increasing levels of UV-B radiation (280-320 nm). In addition ozone in the troposphere is a pollutant and also capable of affecting the photosynthetic machinery. In this study, 5-year-old European beech trees were exposed from 1 July to October 1993 to two levels of UV-B radiation and two levels of ozone, alone and in combination, in open-top chambers equipped with lamps. The simulated UV-B levels corresponded to either clear sky ambient level or a 14% decrease in the stratospheric ozone column over eastem Denmark, resulting in a 23% difference in biologically effective UV-B (UV-BBE) irradiance. The maximum UV-BBE given was 8.61 kJ m"^ day"'. The ozone levels were either the ambient (average 32 nl 1"' ) or ambient with ozone addition (average resulting concentration 71 nl 1"'). Compared to the control treatment (ambient UV-B, ambient O3) the elevated levels of UV-B and O3 affected the trees negatively, expressed as declines in net photosynthesis (Pn), stomatal conductance (gs), chlorophyll fluorescence (Fv/F^) and acceleration of senescence, measured as yellowing of the leaves. The UV-B treatment induced stomatal closure before the other treatments did. The magnitude of the decreases in Pn and F^fF^ occurred in the order: control < UV-B < O3 < UV-B and O3. Compared to the control, the combination treatment with high levels accelerated the visual senescence processes by ca 27 days, while for high UV-B and O3 alone, there was an acceleration by 14 and 21 days, respectively. UV-B and O3 in combination enhanced the negative effects compared with UV-B and O3 alone. The Pn and F^IF^ results could be related to this acceleration process. The chamber effect was investigated by comparing the control plots with a plot without open-top chamber. The trees in the chambers showed a higher Pn and Fy/Fn, and a 14-day delayed senescence compared to the trees outside, probably caused by higher temperatures, a more protected environment and altered conditions inside the chambers.